The invention relates to a magnetic resonance apparatus which is provided with a gradient device which includes:
at least one gradient coil for generating a magnetic gradient field in an imaging volume of the apparatus by means of gradient current pulses,
a power amplifier for applying the gradient current pulses to the gradient coil,
a control circuit which is connected to the input of the power amplifier in order to supply the power amplifier with a control signal representing the gradient current pulses, said control circuit being provided with a signal input for receiving an input signal wherefrom the control signal is derived.
An apparatus of this kind is known from U.S. Pat. No. 5,442,290.
Generally speaking, a medical MRI (Magnetic Resonance Imaging) apparatus is used to form images of an object to be imaged which is situated in an imaging volume of the apparatus in which a uniform, steady field (the so-called main field) exists. A gradient field which varies (usually linearly) as a function of the location is superposed on the main field so as to indicate, in the region to be imaged, the point (x, y, z) which is to be imaged at a given instant. Each point (x, y, z) in the region to be imaged is then indicated by the instantaneous value of an x gradient field, a y gradient field and a z gradient field. The time-dependent variation of these fields is shaped as a pulse, i.e. the so-called gradient pulse, which often has a trapezoidal shape and a duration of the order of magnitude of 1 ms. Said gradient fields are generated by pairs of coils (i.e. one pair for each of the x, y and z co-ordinates), each of which is traversed by associated gradient current pulses.
In the case of digital control of the formation of the gradient pulses in an MRI apparatus, the gradient current pulses generating the pulse-shaped gradient fields are composed of directly successive sub-pulses which will be referred to hereinafter as gradient pulse samples and are produced by a power amplifier which is controlled by an input signal which assumes discrete values only. This input signal can be produced by a converter, for example a pulse width converter (PWM converter) which forms part of a control circuit specifying the appearance of the gradient pulses.
The cited United States patent discloses, notably FIG. 1 and the associated description, an MRI apparatus which includes a gradient device with a gradient coil which is fed by a control loop. The control loop includes a control circuit and a power amplifier in the form of a pulse width modulated power supply source. As is known, PWM power supply sources deliver an output current in the form of a pulse series whose mean value constitutes the desired output current. In addition to the (desired) mean value, the output current also contains undesirable higher harmonics which may have a disturbing effect on the operation of the MRI apparatus. In order to remove such higher harmonics from the output current, a low-pass filter is arranged between the power amplifier and the gradient coil. A control circuit in the form of a comparator is connected to the input of the power amplifier, one input of said comparator receiving a sawtooth reference signal whereas another input receives the time integrated value of the difference between the desired current and the observed current through the gradient coil. In this known configuration the output signal delivered by the comparator constitutes the control signal representing the gradient current pulses whereas the signal applied to the latter input of the comparator is the input signal wherefrom the control signal is derived.
A variety of effects may occur in the control loop of a gradient device, both internally within the loop as externally, with the result that the shape of the gradient current pulse through the gradient coil is not exactly as specified by the input signal applied to the signal input of the control circuit. In order to counteract the effect of such disturbing influences, the current through the gradient coil in the known gradient device is fed back to the signal input of the control circuit. As a result, the actual shape of the gradient current pulse approximates the desired shape more closely. However, a deviation remains which is due to the presence of a variety of capacitive and/or inductive elements in the gradient loop from the signal input for the desired signal up to and including the gradient coil; due to the finite bandwidth of the fed back gradient loop, however, the effect thereof cannot be corrected by feedback.
It is an object of the invention to provide a magnetic resonance apparatus wherein the gradient device more closely approximates the desired shape of the gradient current pulse. To this end, the apparatus according to the invention is characterized in that between the signal input of the control circuit and a point preceding the input of the power amplifier there is inserted a feed forward loop which includes at least one filter whose impedance characteristic is the inverse of the impedance characteristic of at least one of the components traversed by the output current of the power amplifier.
The signal having the desired shape of the gradient current pulse is applied to the input of the feed forward loop, i.e. to the point where the feed forward loop is connected to the signal input. This signal is distorted by said filter having the inverse impedance characteristic; the effect thereof consists in that the distortion caused by said component traversed by the output current of the power amplifier is such that it cancels the former distortion.
The power amplifier in a preferred embodiment of the invention is constructed as a pulse width modulatable amplifier which is succeeded by a power output stage, the output of the feed forward loop being connected to the input of the pulse width modulatable amplifier. Because the output signal of the feed forward loop is applied to a point in the control circuit of the gradient coil where a comparatively low power level exists, the components in the feed forward loop may be constructed as low power types, thus enabling an inexpensive implementation which does not require a large volume.
The filter having the inverse impedance characteristic is constructed so as to be digital in a further preferred embodiment of the invention. It is often very difficult to design an analog filter having an impedance characteristic which is the inverse of a given impedance characteristic. Moreover, due to manufacturing tolerances of the components, the desired filter characteristic may still deviate from the design, so that the desired effect is not attained. These problems are avoided in the case of a digital construction of the inverse filter.
A feedback loop is provided between the gradient coil and the signal input of the control circuit in a further embodiment of the invention. This embodiment enables a first, coarse correction to be made in respect of deviations between the desired and the actual gradient current pulse. The filter in the feed forward loop can then be designed for a smaller signal range; this aspect is of importance notably for a digital filter, considering the width of a variety of registers, for example the number of bits determining the dimension (and hence the speed) of an analog-to-digital converter in the digital filter.
The feedback loop in a further preferred embodiment of the invention includes a difference forming device which is connected so as to succeed the signal input of the control circuit, a delay member being connected between the input of the feed forward loop and the difference forming device. This step offers the following advantageous effect: in the forward loop, like in the direct control loop of the gradient coil and in the feedback loop, a signal delay is introduced relative to the input signal wherefrom the control signal is derived. The signal which is fed back by the feedback loop is compared with said input signal. Due to the delay of the fed back signal, this comparison would still yield a difference in the case of identical signal shapes; this is, of course, undesirable. In order to carry out the comparison correctly, the input signal applied to the signal input of the control circuit is delayed by the same delay as that experienced in the control loop by the signal fed back to the difference forming device. Due to the presence of the delay member ahead of the difference forming device of the feedback loop, therefore, the delay of the various signal paths can be made equal. The magnitude of the signal subsequent to the difference forming device (so the amplitude thereof) is dependent on the magnitude of the difference in the signal delay, notably at the area of the positive going and negative going edge of the usually trapezoidal gradient current pulse. By making this difference zero (meaning that it is significantly reduced in practice), the maximum value of the signal directly subsequent to the difference forming device can be kept small, so that the components succeeding the difference forming device can be constructed so as to have small dimensions.
In a further embodiment of the invention an analog-to-digital converter is connected between the difference forming device and the signal input of the control circuit. In the case of a digital construction of the part of the control loop succeeding the difference forming device, an analog-to-digital converter should be arranged so as to precede this part of the control circuit. For the above-mentioned small amplitude of the signal to be processed by this converter, the number of bits determining the dimension of the analog-to-digital converter can be kept small, so that its speed may be high or, alternatively, its resolution may be higher for the same number of bits.
In a further embodiment according to the invention the magnetic resonance apparatus is provided with at least one low-pass filter which is connected in series with the gradient coil, the feed forward loop in said magnetic resonance apparatus including a first filter whose impedance characteristic is the inverse of the impedance characteristic of the gradient coil and also including a second filter whose impedance characteristic is the inverse of the impedance characteristic of said low-pass filter. The deviations between the desired shape and the actual shape of the gradient current pulse are often caused mainly by the presence of a variety of capacitive and/or inductive effects in said two components. In those cases it is important to counteract the effects thereof by introducing the inverse impedance characteristics in the feed forward loop.